Switching power supplies used as power supplies for operating circuits of electronic units are classified as isolating-type and non-isolating-type switching power supplies. In the case of an isolating-type switching power supply, the switching power supply comprises an input circuit including a first coil on the primary side of a transformer as an input side and an output circuit including a second coil magnetically coupled with the first coil through a core, on the secondary side as an output side.
The input circuit includes the first coil, a DC power supply, a switch device, and a control circuit.
A current supplied from the DC power supply to the first coil flows intermittently by means of ON/OFF control of the switch device performed by the control circuit. As a result, an AC voltage is generated in the second coil, which is magnetically coupled with the first coil. This AC voltage is extracted as a DC output voltage by a rectifying circuit and a smoothing circuit provided for the output circuit, and supplied to a load.
A pulse-width-modulation IC (hereinafter called a PWM IC) is usually used for the control circuit. The PWM IC is basically formed of an error amplifier, a triangular-wave generator, and a comparator.
An output voltage Vout from the output circuit is fed back to the inverted input terminal of the error amplifier and a constant reference voltage is applied to the non-inverted input terminal. The triangular-wave generator outputs, for example, a fundamental triangular wave having a frequency of 20 kHz to 2 MHz. The fundamental triangular wave is input to the inverted input terminal of the comparator and an amplified voltage Vamp amplified by the error amplifier is input to the non-inverted input terminal. The comparator compares the fundamental triangular wave with the amplified voltage Vamp and generates a driving pulse for ON-control of the switch device during the period when the fundamental triangular wave is larger than the amplified voltage Vamp. Therefore, as shown in FIGS. 1A to 1C, when the amplified voltage Vamp1 is large, the pulse width of the driving pulse becomes small, and when the amplified voltage Vamp2 is small, the pulse width of the driving pulse becomes large. As a result, the ON period of the switch device is varied and pulse-width-modulation control is constantly applied to the output voltage Vout from the output circuit. However, the conventional PWM control method can only feed back one output voltage to the PWM IC. In other words, it can only assure output of one stable output voltage. Therefore, the conventional PWM control method does not suit a switching power supply requiring a plurality of output voltages.
On the other hand, the use of “resonant” switching techniques has been employed in the art to reduce or eliminate the switching losses caused by high switching frequencies. Resonant switching techniques generally comprise the inclusion of an LC subcircuit in series with a semiconductor switch which, when turned ON, creates a resonating subcircuit within the converter. Timing the ON/OFF control cycles of the resonant switch to correspond with particular voltage and current conditions across respective converter components during the switching cycle inherently reduces or eliminates many frequency-dependent switching losses. However, the ON/OFF switch may cause a high magnitude current in the transformer, which can reduce the transferring efficiency.